Apple wireless keyboard/mouse with 1.2V NiMH batteries

"I don't have specific experience with those products, but the type of battery you purchased is designed to do exactly what you intend. If you check the voltage of an alkaline battery after it's been used a while, you'll find that its voltage will have dropped below 1.5v. In fact, the voltage drops off faster for them than for NiMH batteries.

From the Wikipedia article on rechargeable batteries:

Also note that most NiMH AA or AAA batteries rate their cells at 1.2 volts. However, this is not a problem in most devices because alkaline batteries drop in voltage as the energy is depleted. Most devices will still operate with a voltage between 0.9 and 1.1 volts.

Here are a few articles to give you more information:

Why are rechargeable AA batteries 1.2v?
Adventures in rechargeable batteries - Jeff Atwood on capacity and charging
Batteries Explained"

It's fine using Ni-MH batteries with this product, but be sure, you choose type with low self discharge, because some rechargeable batteries will self discharge in about 1 month. I am using Sanyo Eneloop (other companies make low self discharge batteries too). They are not so high capacity comparing to other brands, but when you use them after one year they are still full charged.